a) Field of the Invention
The present invention relates generally to a vehicular automatic transmission equipped with a failure detecting apparatus. The present invention, more particularly, relates to a technique on a hydraulic circuit of the vehicular automatic transmission of an engaging pressure electronically controlled type in which the hydraulic circuit is simplified, the number of assembled parts are reduced, and a reduction in size of a control valve body is achieved.
b) Description of the Related Art
A Japanese Patent Application First Publication No. Heisei 8-121586 published on May 14, 1996 exemplifies a previously proposed hydraulic control apparatus for the vehicular automatic transmission of the engaging pressure electronically controlled type.
In the previously proposed vehicular automatic transmission disclosed in the above-identified Japanese Patent Application Publication, as a fail-safe valve of an LR (low-and-reverse) brake which is clutched when a gear range is a first-speed D (Drive) range and is released when the gear range is a second-speed, a third-speed, or a fourth-speed D range, a forceful hydraulic drain structure is disposed which forcefully drains the hydraulic supplied to the LR brake at a time of the gear range is in the D range second-speed, third-speed, and fourth-speed at which the hydraulic in at least one of either a 2-nd brake pressure P2ND or an OD (Overdrive) clutch pressure POD is developed, the 2-nd brake pressure P2ND being the hydraulic for a 2-nd-brake to be clutched at the time of D range 2-nd or 4-th speed and the OD clutch pressure POD being the hydraulic for an OD clutch to be engaged at the time of D range 3-rd or 4-th speed.
However, in the previously proposed hydraulic control apparatus for the vehicular automatic transmission, no detection means is provided to detect whether a first spool which constitutes the forceful hydraulic drain structure is stuck (or a sticky slip occurs). If the first spool is stuck at a position where the OD clutch pressure POD enters, the LR brake would be engaged and would be interlocked due to the stick on the first spool if the abnormality occurs in a hydraulic system on the LR brake pressure and the undesired hydraulic is developed.
It is, hence, an object of the present invention to provide the vehicular automatic transmission of the engaging pressure electronically controlled type with which a failure detecting apparatus is equipped and the failure detecting apparatus can achieve such a fail-safe operation as to forcefully drain an engagement element pressure at a most appropriate timing neither giving an ill effect on a gear shift control nor developing the interlock under an occurrence of the failure and which can check to see if this fail-safe operation is always achieved without failure.
The above-described object can be achieved by providing a vehicular automatic transmission, comprising: a plurality of engaging elements; a shift gear mechanism in which combinations of engagement and release of the respective engaging elements are selected by means of hydraulic control section so as to perform a multiple range of gear shift; a hydraulic switching device that is installed in the hydraulic control section to prevent the shift gear mechanism from being interlocked; and a failure detecting apparatus to detect an occurrence of failure of the hydraulic switching device, the failure detecting apparatus including: a gear range determinator to determine in which gear range the present gear range of the automatic transmission falls; a failure detection start determinator to determine a start of the failure detection of the hydraulic switching device according to a result of determination by the gear range determinator; a memory storing parts of the engaging elements which are selected as the combinations of engagement for the respective gear ranges and other parts than those for the respective gear ranges in a table form; a hydraulic switching device checking device to intentionally output an engagement command signal to command to engage to each of the other parts of engaging elements which are not to be engaged and which are read from the memory; and a hydraulic switching device abnormality state determinator to determine through the checking device whether the hydraulic switching device is in an abnormal state depending on whether the hydraulic is transmitted to a corresponding one of the other parts of engaging elements in response to the engagement command signal.